1. Field of the Invention
The present invention relates to a developing device for developing a latent image with a developer to thereby produce a corresponding toner image, an image forming process unit using the developing device, and an image forming apparatus using either one of the developing device and image forming process unit.
2. Description of the Background Art
A copier, printer, facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus generally includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A latent image is formed on the image carrier in accordance with image data. A developing device develops the latent image with a developer to thereby produce a corresponding toner image. Today, a magnet brush type developing system using a two-ingredient type developer, or toner and carrier mixture, is predominant over the other developing systems. The toner and carrier mixture is desirable from transferability, halftone reproducibility, and stability against varying temperature and humidity. In this type of developing system, the developer rises on a developer carrier in the form of brush chains. In a developing region where the developer carrier faces the image carrier, the toner is transferred from the developer to the latent image carried on the image carrier. The brush chains contact the latent image in the developing region.
The developer carrier is usually made up of a hollow, cylindrical sleeve and a magnet roller disposed in the sleeve for forming a magnetic field. Carrier grains rise on the sleeve along magnetic lines of force issuing from the magnet roller. Charged toner grains deposit on the carrier grains, forming a magnet brush. The magnet roller has a plurality of magnetic poles each being implemented by a, e.g., a rod-like magnet. Among them, a main pole adjoins the developing region for causing the developer to form the magnet brush. At least one of the sleeve and magnet roller rotates, conveying the developer risen on the sleeve. In the developing region, the main pole causes the developer to rise along its magnetic lines of force. The brush chains contact the surface of the image carrier while yielding. At this instant, the brush chains rub themselves against the latent image due to a difference in linear velocity between the image carrier and the sleeve, feeding the toner to the latent image.
Japanese Patent Laid-open Publication No. 7-84439, for example, discloses an image forming apparatus using the above-described developing device and a low-potential system. The low-potential system lowers the charge potential of the image carrier to 400 V or below in order to reduce the electrostatic fatigue of the image carrier ascribable to repeated charging and exposure. This successfully extends the life of the image carrier.
The image carrier is apt to suffer from serious hazard when initially charged. Particularly, in a charging system using, e.g., a scorotron charger, charged particles derived from discharge directly fall on the image carrier, accelerating the deterioration of the image carrier due to ionization. The low-potential system is effective against such an occurrence as well.
Further, it is likely that a potential difference between the image carrier and a casing or similar member adjoining it exceeds a discharge start voltage. Discharge between the image carrier and the member adjoining it would adversely effect image quality. However, when the charge potential of the image carrier is as low as 400 V or below, the above potential difference can be reduced below a value represented by a Patchen's curve, obviating the adverse influence of discharge on image quality.
On the other hand, in the low-potential system, a development potential is lowered along with the charge potential of the image carrier. The development potential refers to the absolute value of a potential difference between the potential of the exposed portion of the image carrier and a bias applied to the developer carrier. It is therefore necessary to increase the developing ability of the developing device, so that target image density is achievable with a development potential lower than conventional. To increase the developing ability, the end of an effective developing electrode, which faces the image carrier, may be brought closer to the surface of the image carrier. Alternatively, the amount of charge to deposit on the toner may be reduced.
A decrease in development potential, however, gives rise to the following problems. The development potential varies due to the variation of the charge potential of the image carrier or that of the quality of light for forming a latent image. When the development potential is lowered, the variance of the potential increases relative to the absolute value of the potential. The amount of charge or the quantity of light varies due to the contamination of a charging member included in a charger or that of optics and varies with respect to time or space. An increase in the variance of the development potential aggravates its influence on image density. As a result, the low-potential system is more likely to lower image density or render image density irregular than the conventional high-potential system.
Particularly, to implement target image density with the low development potential, it is necessary to increase so-called a γ value for development. The γ value refers to the slope of the rising portion of a development characteristic curve representative of image density varying in accordance with the rise of the development potential. For this purpose, some different methods are available, e.g., one that reduces the electric resistance of magnetic carrier grains, one that increases the dielectric constant of the grains, one that reduces a gap for development, one that increases the linear velocity ratio of the developer carrier to the image carrier, and one that reduces the amount of charge of the toner. However, an increase in γ value makes image density more susceptible to the influence of the variation of the development potential, compared to the conventional high-potential system. Moreover, an increase in γ value is apt to deposit an excessive amount of toner on the image carrier because the magnet brush contains a sufficient amount of toner. The excessive amount of toner is greater than the minimum amount implementing saturation reflection density after fixation of a toner image on a sheet. Excessive toner deposition is therefore apt to bring about background contamination, toner scattering at the time of transfer, smearing of a line image and other defects.
Japanese Patent Laid-Open Publication No. 2000-305360 teaches a developing device using a toner and carrier mixture and constructed to insure desirable images over the entire density range. For this purpose, the developing device satisfies at a high level a developing condition for increasing image density and a developing condition for implementing a desirable low-contact image.
The key to a high quality, long life image forming apparatus is the extension of the life of the developer and faithful image formation. A developer is subjected to mechanical hazard due to its contact with magnetic grains or a metering member. More specifically, it is likely that an additive coating the individual grain is buried in the grain and lowers the fluidity or the charging ability of the grain. It is therefore extremely difficult to maintain desirable image quality. While the mechanical hazard may be reduced at the time of charging of the developer, this prevents the amount of charge from sufficiently increasing at the time of frictional charging. To form a high-definition image, the difference between the charge and the potential after exposure may be reduced as far as possible while the optics may write a latent image with as low energy as possible. In this case, the precondition is that because the potential contrast decrease, use is made of relatively low-charge toner for increasing the amount of development, i.e., increasing the developing ability.
In the event of image transfer, toner scattering occurs little because the potential of a latent image and that of the background are relatively low. Freeing the image carrier from deterioration is the effective implementation for extending the life of the image carrier. The image carrier is subjected not only to the previously mentioned optical fatigue, but also to serious hazard at the time of initial charging. This is particularly critical when use is made of a scoroton charger, as stated earlier. In light of this, it has been proposed to halve the conventional initial charge of −800 V to −400 V. However, selecting a low potential, i.e., absolutely lowering the charge potential in a negative-to-positive development system simply means lowering the development potential inclusive of the bias condition. It is therefore necessary to increase the developing ability for thereby lowering the saturation development potential. However, the problem is that a low-potential process is susceptible to the variation of the surface potential of the image carrier. This is because the absolute value of the charge potential is originally so low, the influence of irregularity is noticeable.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-19588, 5-19601, 6-102767, 11-295925, 2000-66490 and 2001-60015.